It is common for a tensioner such as a belt tensioner to have a means to dampen movement of the tensioner arm caused by belt tension fluctuation. The required magnitude of this damping depends on many drive factors including geometry, accessory loads, accessory inertia, engine duty cycle and others. For instance, drive systems that have higher torsional input or certain transient dynamic conditions may require higher damping to sufficiently control tensioner movement. Although higher damping is very effective at controlling arm movement, it can also be detrimental to other critical tensioner functions (e.g., slow or no response to slack belt conditions). In addition, variation or change in damping that occur as a result of manufacturing variation, operating temperature and component break-in or wear can also cause the tensioner to be unresponsive.
Timing belt systems have benefited from the use of asymmetric damping to address this problem. An asymmetrically damped tensioner provides damping when additional belt tension is encountered, but is free to respond to slack belt conditions. Although asymmetric functionality may not be required for all other front end accessory drive tensioners, the potential for increased service life, solving other transient dynamic system problems including belt slip, or simply making the tensioner less sensitive to damping variation make it a desirable design option.
Many belt tensioner damping mechanisms that utilize frictional damping use axial forces to move components of the tensioner to create the frictional force that does the damping. These designs tend to require a means to contain the axial force and some components of the belt tensioner must be more robust to withstand the axial force over the lifetime of the tensioner.
One example of axial damping is the use of a Bellville spring, disc spring, or wave washer acting perpendicular to a torsional spring. The disc spring forces surfaces together to create rotational friction, thereby damping movement of the belt tensioner. These springs are often limited by package space, and therefore may have inadequate damping magnitude. Alternatively, the damping magnitude may be sufficient, but extremely sensitive to spring preload, thereby requiring highly accurate dimensional tolerances to maintain the desired preload on rotating friction surfaces. Wear on the system caused by this rotational friction may therefore skew the axial damping force away from the preferred tolerance.
Another example of axial damping is the use of a coil spring providing damping force in additional to torsional biasing of the belt tensioner towards the belt. In addition to the problems described above with respect to the disc spring or Bellville spring, axial damping may exhibit varying damping depending on the position of the tensioner and amount of stress on the coiled spring due to the winding or tensioning rotation of the arm, and thus coil spring.
Tensioner damping that is unequal, or asymmetric, has been shown to provide superior control of tensioner arm movement compared to typical symmetrical systems. An asymmetrically damped tensioner provides damping when additional belt tension is encountered but is free to respond to slack belt conditions, thereby increasing resistance to undesirable tensioner movement and freely allowing desirable tensioner movement to maintain tension in the belt.